Thermal transfer printing, electrophotography, ink-jet printing, etc. are comprehensively investigated in connection with color hard copy technology. Among them, thermal transfer color donation has many advantages in comparison with other methods in view of the ease of maintenance and operation of an apparatus, the low prices of the apparatuses and the expendables, etc.
The thermal transfer printing is classified into two methods: (1) a method in which a thermal transfer dye donating material having a thermally fusible ink layer formed on a base film is heated by a thermal head to melt the ink, and thus recording is conducted on a thermally transferred image receiving material for thermal transfer printing; and (2) a method in which a thermal transfer dye donating material having a color material layer formed on a base film and containing a thermally migrating dye is heated by a thermal head to transfer thermally the dye onto the image receiving material for thermal transfer printing. The latter method, namely the thermal migration transfer printing is especially advantageous in full color recording in high quality because the transfer concentration of the dye can be varied by varying the energy given to a thermal head, thus facilitating gradation recording. The thermally migrating dye, however, is restricted in many respects, and few dyes can satisfy all the characteristics required.
The characteristics required are, for example, spectrographic properties suitable for color reproduction, high thermal migration, high light-fastness, high heat stability, high resistance to various chemicals, resistance to sharpness deterioration, resistance to retransfer, ease of the synthesis, ease of preparation of the thermal transfer dye donating material, and so on. Accordingly, the development of cyan dyes satisfying these requirements are especially desired.
A variety of cyan dyes are proposed. Among them, indoaniline type dyes are relatively satisfactory and are described in JP-A-60-239289, JP-A-61-22993, JP-A-61-268493, JP-A-62-191191, and JP-A-63-91287. (the term "JP-A" as used herein means an "unexamined published Japanese patent application") Nevertheless, these dyes cannot satisfy all the requirements of spectrographic absorption suitable for color reproduction, transfer concentration, light-fastness and retransfer of images.